Hello,

Did you completely remove Norton using removal tool before you installed AVG?

If you have trial version installed you are entitled for the full support here:
http://www.avg.com/ww-en/customer-support

Otherwise, CPU usage could have been caused by caching of the files or due to conflict with other software.

Please try to disable cache server:
Tools - Advanced settings - Cache server - Untick caching enabled.

If that does not help, provide us with list of programs that are running in background.

Thanks